The development of highly absorbent articles for blood and blood-based fluids such as catamenial pads (e.g., sanitary napkins), tampons, wound dressings, bandages and surgical drapes can be challenging. Compared to water and urine, blood and blood based fluids such as menses are relatively complex mixtures of dissolved and undissolved components (e.g., erythrocytes or red blood cells). In particular, blood-based fluids such as menses are much more viscous than water and urine. This higher viscosity hampers the ability of conventional absorbent materials to efficiently and rapidly transport these blood-based fluids to regions remote from the point of initial discharge. Undissolved elements in these blood-based fluids can also potentially clog the capillaries of these absorbent materials. This makes the design of appropriate absorbent systems for blood-based fluids such as menses particularly difficult.
In the case of catamenial pads, women have come to expect a high level of performance in terms of comfort and fit, retention of fluid, and minimal staining. Above all, leakage of fluid from the pad onto undergarments is regarded as totally unacceptable. Improving the performance of such catamenial pads continues to be a formidable undertaking, although a number of improvements have been made in both catamenial structures, and materials used in such structures. However, eliminating leakage, particularly along the inside of the thighs, without compromising fit and comfort, has not always met the desired needs of the consumer.
The users of sanitary napkins, and the like, have also come to expect the surface of such products to provide a cleaner, more sanitary and drier aspect than common cloth or nonwoven materials have historically provided. Current sanitary napkin products are typically provided with nonwoven or formed-film permeable topsheets that are designed to move discharged menstrual fluids rapidly through and into an underlying absorbent structure. This rapid movement of acquired menstrual fluids is designed to provide a drier and cleaner surface adjacent the wearer of the product.
The absorbent structures of current catamenial (e.g., sanitary napkin) pads have typically comprised one or more fibrous layers for acquiring the discharged menstrual fluid from the permeable topsheet and distributing it to an underlying storage area. Absorbent structures for relatively thin versions of prior catamenial products usually comprise a fluid acquisition layer (often called a "secondary topsheet") that is adjacent to the permeable topsheet. This "secondary topsheet" typically is made from an air-laid-tissue web or a synthetic nonwoven. Underlying this secondary topsheet is the main absorbent core that is typically made from air-laid or wet-laid tissue. The absorbent core often contains a particulate absorbent gelling material that can be encased or enveloped within this tissue. Such encased or enveloped cores are often referred to as tissue laminate cores. See, for example, U.S. Pat. No. 4,950,264 (Osborn), issued Aug. 21, 1990 and U.S. Pat. No. 5,009,653 (Osborn), issued Apr. 23, 1991, that disclose tissue laminate cores used in sanitary napkin products.
Prior catamenial absorbent structures made from fibrous layers have a number of problems. One is the difficulty in ensuring adequate topsheet dryness. In particular, the acquired menstrual fluid can potentially leak back through the main topsheet. This phenomenon is often referred to as "rewet." Rewet can be significantly reduced by increasing the fluid capillary pressure exerted by the absorbent core for fluid relative to the main and secondary topsheet. The greater the disparity in fluid capillary pressure between core and topsheet elements, the greater the potential for providing a dry topsheet surface in contact with the body. This potential, however, can only be realized if the kinetics of fluid movement throughout the core is sufficiently fast.
Prior catamenial absorbent structures, and in particular catamenial pads using such structures, have also had a greater chance of causing panty and body soiling. This is because the absorbent structure lacks resilience, leading to bunching of the pad. This lack of resilience, and consequent bunching, has also caused these prior catamenial pads to provide poorer fit and comfort for the user.
An alternative to conventional catamenial absorbent structures are absorbent foams. Absorbent foams can possess desirable wet integrity, can provide suitable fit throughout the entire period the article is worn, and can minimize changes in shape during use (e.g., uncontrolled swelling, bunching, etc.). In addition, catamenial products containing such foam structures can be easier to manufacture on a commercial scale. For example, absorbent cores can simply be stamped out from continuous foam sheets and can be designed to have considerably greater integrity and uniformity than conventional absorbent fibrous webs. Such foams can also be prepared in any desired shape, or even formed into single-piece catamenial pad, or other absorbent article used to absorb blood or blood-base fluids such as tampons, wound dressings, bandages and surgical drapes.
Foams of various types have been suggested for use in tampons, sanitary napkins and other articles that absorb blood and blood-based fluids. See for example U.S. Pat. No. 4,110,276 (DesMarais), issued Aug. 29, 1978 (soft, flexible, open celled foams made from polyurethanes, cellulose, or styrene/butadiene rubber that can be used in tampons and sanitary pads); U.S. Pat. No. 4,752,349 (Gebel), issued Jun. 21, 1988 (foams of "medium cell size" hydrophilized by surfactant treatment and having a density within the range of 0.1 to 0.8 g/cc); U.S. Pat. No. 4,613,543 (Dabi), issued Sep. 28, 1986 (hydrophilic cellular polymers used in catamenial products); U.S. Pat. No. 3,903,232 (Wood et al.), issued Sep. 2, 1975 (compressed hydrophilic polyurethane foams useful in biomedical applications, including catamenial devices); U.S. Pat. No. 4,049,592 (Marans et al.) issued Sep. 20, 1977 (biodegradable hydrophilic polyurethane foams highly absorptive upon contact with liquids or bodily fluids having utility in sanitary napkins and the like). Prior foams used in these products have tended to have relatively large cell sizes. As a result, these prior foams do not exert sufficient fluid capillary pressure for blood and blood-based fluids to acquire discharged menstrual fluids quickly from and through the topsheet of catamenial products such as sanitary napkins. This results in undesirable rewet since the surface in immediate contact with the body retains some of the fluid that is not absorbed into the core and is available to be transferred back onto the body of the wearer.
Suitable absorbent foams for absorbent products have also been made from High Internal Phase Emulsions (hereafter referred to as "HIPE"). See, for example, U.S. Pat. No. 5,260,345 (DesMarais et al), issued Nov. 9, 1993 and U.S. Pat. No. 5,268,224 (DesMarais et al), issued Dec. 7, 1993. These absorbent HIPE foams provide desirable fluid handling properties, including: (a) relatively good wicking and fluid distribution characteristics to transport fluid away from the initial impingement zone and into the unused balance of the foam structure to allow for subsequent gushes of fluid to be accommodated; and (b) a relatively high storage capacity with a relatively high fluid capacity under load, i.e. under compressive forces. These HIPE absorbent foams are also sufficiently flexible and soft so as to provide a high degree of comfort to the wearer of the absorbent article; some of these foams can be made relatively thin until subsequently wetted by the absorbed body fluids. See also U.S. Pat. No. 5,147,345 (Young et al), issued Sep. 15, 1992 and U.S. Pat. No. 5,318,554 (Young et al), issued Jun. 7, 1994, which disclose absorbent cores having a fluid acquisition/distribution component that can be a hydrophilic, flexible, open-celled foam such as a melamine-formaldehyde foam (e.g., BASOTECT made by BASF), and a fluid storage/redistribution component that is a HIPE-based absorbent foam.
HIPE foams can provide the fluid capillary pressure necessary to remove most of the menstrual fluid from the body, or topsheet adjacent to the body, thus minimizing rewet. However, it has been found that the residual hydratable salts such as calcium chloride typically present in prior HIPE foams can impair the rapid acquisition blood and blood-based fluids by these foams, and especially the wicking of such fluids within these foams. As noted above, blood and blood-based fluids such as menses are more highly viscous than water and especially urine. The higher viscosity of these fluids is further increased by the presence of these salts. Moreover, prior HIPE foams typically have a foam microstructure too small to admit readily the undissolved components of blood and blood-based fluids such as red blood cells.
Accordingly, it would be desirable to be able to make an open-celled absorbent polymeric foam material, in particular an absorbent HIPE foam, that: (1) can rapidly absorb blood and blood-based fluids such as menses; (2) can be used as absorbent members for relatively thin catamenial pads (e.g., sanitary napkins) and other catamenial products such as tampons, as well as wound dressings, bandages, surgical drapes and the like; (3) allow storage components having higher capillary or osmotic absorption pressures to partition away this fluid; (4) keep the source of the blood-based fluids relatively free of rewet, even in "gush" situations and under compressive load; (5) are soft, flexible, resilient, and comfortable to the wearer of the absorbent article, and (6) have a relatively high capacity for fluid to provide efficient in their utilization of costly components.
While thin catamenial products are desired by many users, there is significant demand for relatively thick products. For example, a thick product may provide a perceived ability to better absorb and retain fluid. Also, a thick product may offer improved fit. It would therefore be desirable to have a relatively thin absorbent foam material(s) as the absorbent core of a catamenial product that allows the use of inexpensive filler materials (e.g., airfelt) to provide bulk/thickness.